1. Field of Invention
The present invention relates to a fuel injection amount control method and apparatus for an internal combustion engine and, more particularly, to a fuel injection amount control method and apparatus for an internal combustion engine having a plurality of cylinders or cylinder groups to which air supplied from a common intake passage is distributed.
2. Description of Related Art
In general, in controlling a fuel injection amount of an internal combustion engine, a fuel injection amount suited for an operating state of the engine detected by respective sensors is first calculated. This fuel injection amount is calculated as a time for supplying electricity to injectors, that is, a time for fuel injection. Based on the time for fuel injection, the injectors are driven so that the amount of fuel supplied to the engine is controlled.
In calculating the aforementioned time for fuel injection, the mass G of air per unit time is detected by the air flow meter, the rotational speed N of the engine is detected, for example, by the crank angle sensor, and the mass G of air per unit time is divided by the rotational speed N of the engine so as to obtain a mass G/N of air per rotation of the engine. Based on the mass G/N of air per rotation of the engine, a basic injection time is calculated. As shown in FIG. 6, this basic injection time has a linear relationship with the mass G/N of air per rotation of the engine. When the reference mass of air per rotation of the engine is equal to (G/N)0, the reference injection time is equal to KINJ. The reference mass (G/N)0 of air per rotation of the engine and the reference basic injection time KINJ are constants that are preliminarily determined according to a characteristic of the engine. Therefore, when the mass G/N of air per rotation of the engine is equal to G/N, the basic injection time is calculated according to a formula shown below. EQU basic injection time=((G/N)/(G/N)0).times.KINJ
If a load factor kl is defined as the mass G/N of air per rotation of the engine divided by the reference mass (G/N)0 of air per rotation of the engine, the basic injection time can also be expressed as shown below. EQU basic injection time=kl.times.KINJ
For example, in the case of a V-type internal combustion engine wherein external air is distributed to the respective banks from a common intake passage and fuel is supplied from injectors provided separately for the respective banks, the aforementioned basic injection time is calculated as a time for injecting fuel from the injectors of both the banks. Therefore, half of the mass G of air per unit time, which is detected by the air flow meter provided in the common intake passage, is distributed to each of the banks. In other words, air of the mass 0.5G is distributed to each of the banks. Besides, the load factor for each of the banks is defined as 0.5kl. The basic injection time for each of the injectors is then calculated as follows. EQU basic injection time=0.5kl.times.KINJ
The fuel injection time for each of the injectors is calculated by correcting the aforementioned basic injection time for each of the banks in accordance with an operating state of the engine.
One of the banks is referred to as a first bank, and the other is referred to as a second bank. The fuel injection time, the air-fuel ratio feedback correction factor and the air-fuel ratio learning value for the first bank are denoted by tau (1), FAF (1) and KGi (1) respectively. The fuel injection time, the air-fuel ratio feedback correction factor, and the air-fuel ratio learning value for the second bank are denoted by tau (2), FAF (2) and KGi (2) respectively. The correction factor and the invalid injection time that can be commonly used for both the banks are denoted by K and TAUV respectively. Then, the fuel injection times tau (1) and tau (2) are calculated using formulas shown below. EQU tau(1)=0.5kl.times.KINJ.times.FAF(1).times.KGi(1).times.K+TAUV EQU tau(2)=0.5kl.times.KINJ.times.FAF(2).times.KGi(2).times.K+TAUV
The air-fuel ratio feedback correction factors FAF (1) and FAF (2) are calculated separately for the respective banks in accordance with values detected by oxygen sensors that are provided in exhaust passages of the respective banks. Through correction of a fuel injection time based on the air-fuel ratio feedback correction factors FAF (1) and FAF (2), feedback control for making the air-fuel ratio close to a stoichiometric air-fuel ratio is performed. Such feedback control of the air-fuel ratio aims at optimally purifying combustion gas discharged from the respective banks in a three-way catalytic converter provided in the exhaust passage of the engine.
The air-fuel ratio learning values KGi (1) and KGi (2) are calculated separately for the banks, and initially set to "1.0". In order to correct a deviation in air-fuel ratio resulting from differences among bodies of the engine, time variability and conditions for use, these learning values KGi (1) and KGi (2) are calculated based on the aforementioned air-fuel ratio feedback correction factors FAF (1) and FAF (2), for respective learning zones that are classified according to a mass G of air.
The correction factor K is used to correct effects of a transient state or the like. The invalid injection time TAUV is added for correction as an operation delay time that starts with the supply of a driving signal and ends with actual injection of fuel from the injector.
Based on the fuel injection times tau (1) and tau (2) calculated separately for the banks, the injectors of the respective banks are driven so that the fuel injection amount is controlled.
As a prerequisite of calculation of the fuel injection times tau (1) and tau (2) for the respective banks in the aforementioned V-type internal combustion engine, half of the mass G of air per unit time that is detected by an air flow meter provided in a common intake passage is distributed to each of the banks. In other words, air of the mass 0.5G needs to be distributed to each of the banks. However, the inventors of the present invention have confirmed that the mass of air distributed to the first bank differs from the mass of air distributed to the second bank depending on the operating state of the engine. Therefore, if air of the mass 0.55G is distributed to the first bank and air of the mass 0.45G is distributed to the second bank, the basic injection time for the first bank is calculated as a time shorter than a time that should actually be calculated, and the basic injection time for the second bank is calculated as a time longer than a time that should actually be calculated. In the case where the basic injection time for each of the banks has been thus calculated, control patterns of a fuel injection amount will be described with reference to time charts shown in FIGS. 7A to 7B and FIGS. 8A and 8B. As shown in FIGS. 7A and 7B, the mass G/N of air per rotation of the engine is constant, and only the rotational speed N of the engine changes (increases) after a time t11.
In such a case, as indicated by a solid line and an alternate long and short dash line in FIG. 7A, the mass (G/N)1 of air per rotation of the engine for the first bank is maintained at 0.5 G/N before the time t11, and increases with lapse of time after the time t11. Further, as indicated by the solid line and a broken line in FIG. 7A, the mass (G/N)2 of air per rotation of the engine for the second bank is maintained at 0.5 G/N, and decreases with lapse of time after the time t11 in a manner corresponding to the increase in mass (G/N)1 of air.
Since the mass G/N of air per rotation of the engine is constant, the basic injection time for each of the banks as calculated based on the aforementioned formula assumes a constant value of 0.5kl.times.KINJ (See FIG. 7C). Therefore, the basic injection time for the first bank is calculated as a time shorter than a time that should intrinsically be calculated, with respect to a mass (G/N)1 of air for the first bank, which mass increases after the time t11. The second injection time is calculated as a time longer than a time that should intrinsically be calculated, with respect to a mass (G/N)2 of air for the second bank, which mass decreases after the time t11.
If feedback control of the air-fuel ratio is performed in such a case, the fuel injection time for the first bank is corrected afterwards in response to an increase in mass (G/N)1 of air for the first bank. Accordingly, as shown in FIG. 7D, the value OX1 detected by the oxygen sensor provided in the exhaust passage of the first bank is detected on the lean side from the time t11 to a time t12 which is relatively far apart from the time t11. From the time t11 to the time t12, the detected value OX1 is smaller than a reference value corresponding to the stoichiometric air-fuel ratio. As shown in FIG. 7E, the air-fuel ratio feedback correction factor FAF (1) for the first bank keeps increasing until the time t12.
On the other hand, the basic injection time for the second bank is also corrected afterwards in response to a decrease in mass (G/N)2 of air for the second bank. Accordingly, as shown in FIG. 8A, the value OX2 detected by the oxygen sensor provided in the exhaust passage of the second bank is detected on the rich side from the time t11 to a time t13 which is relatively far apart from the time t11. From the time t11 to the time t13, the detected value OX2 is greater than a reference value corresponding to the stoichiometric air-fuel ratio. As shown in FIG. 8B, the air-fuel ratio feedback correction factor FAF (2) for the second bank keeps decreasing until the time t13.
Thus, if the air-fuel ratio remains deviated from the stoichiometric air-fuel ratio toward the lean side (the first bank) or the rich side (the second bank) for a long time, the combustion gas discharged from the respective banks may not be sufficiently purified in the three-way catalytic converter.
The continuous deviation from the stoichiometric air-fuel ratio as described above has an effect on calculation of the aforementioned air-fuel ratio learning values KGi(1) and KGi(2). Therefore, an error may occur in the learning process.
Furthermore, in addition to the feedback control of the aforementioned air-fuel ratio, the control of the fuel injection amount is performed by correcting the aforementioned basic injection time in accordance with an operating state of the engine. Thus, it may not be possible to obtain an appropriate air-fuel ratio or to ensure the operability of the engine in accordance with the operating state of the engine.
In order to prevent the air supplied from the common intake passage from being distributed non-homogeneously to the respective banks, an air flow meter may be provided in each of the banks so that the mass of air can be detected separately for the banks. In this case, however, another problem such as a rise in manufacturing costs occurs.
Further, in the case of an engine control apparatus disclosed in Japanese Patent Application Laid-Open No. HEI 10-9020, feedback control of an air-fuel ratio is performed in consideration of non-homogeneous distribution of purge gas to left and right banks. However, this apparatus does not take any measures against the non-homogeneous distribution of air from the common intake passage to the respective banks (the first and second banks).